The present invention relates to an electrical circuit and particularly one for converting bipolar analog signals to a digital format.
In the processing of analog information from sensors, frequently, the analog signal is a bipolar signal having an asymmetric waveform. Signals, for example, from tachometers employed in jet aircraft engine monitoring systems typically will provide a generally AC waveform in which each half-cycle of bipolar signal may vary in duration such that the positive half-cycle may be longer or shorter than the negative half-cycle. Also, the wave shape typically can be irregular and in addition, contain noise frequency components. Although these signals are adequate for providing a visual display of the general engine speed, the signals lack the accuracy to be processed by conventional circuits for providing extremely accurate engine speed information which is necessary in certain applications.
One such application of engine speed is the utilization of such information to determine when engine maintenance is required. Thus, for example, in the General Electric J85 turbine engine, periodic maintenance is performed as a function of the use of the engine as measured by the number of engine speed excursions through predetermined zones of engine r.p.m.'s. In order to accurately provide engine r.p.m. information which can be employed to provide engine maintenance information, it is desirable, therefore, to supply accurate data as to the engine speed.
This application of engine r.p.m. signal information for predicting engine life maintenance is described in a Pratt & Whitney Aircraft document dated Sept. 16, 1971, entitled Adaptation of the Howell TTRI for J57-P-420 Cumulative LCF Damage Determination and is also described in U.S. Pat. No. 4,031,366 which also describes a hard wired logic signal processing system coupled to an r.p.m. generator. The conversion of the periodic waveforms provided by the tachometer disclosed is an AC to DC converter by which a DC level is provided directly from the AC signal voltage.
Another technique for processing tachometer signals is to simply count the tachometer pulses during a prescribed time period to determine r.p.m. information. Also, AC tachometer signals have been converted to DC through a frequency to voltage conversion circuit as suggested in U.S. Pat. Nos. 3,750,465; 3,686,484 and 3,584,507. An inherent deficiency in such systems is that the signal information can vary rapidly with time, and an error can result due to the relatively slow response of the frequency to voltage conversion circuitry.